Patterns of Inheritance
• Mid-1800s
• Austrian Monk, Gregor Mendel
• Used pea plants to study how traits are
passed from one generation to the next.
• “Basic principles of Heredity”
Mendelian Genetics:
• For his experiments on inherited traits,
Mendel used pea plants
• Because they could produce purebred
individuals (genetically identical)
• How? Self-pollination
• Once he produced Pure-bred strains, he
began to interbreed them in a controlled
environment.
• Mendel crossed pure-bred GREEN-POD
(dominant) plants with pure-bred
YELLOW-POD (recessive) plants.
• The results are shown below:
Why did all the offspring come out green, if one of
it’s parents were yellow?
• Pure-bred= Homozygous= Parent
generation
Parent 1- Homozygous Green (GG)
Parent 2- Homozygous Yellow (gg)
• When they were interbred, each parent
contributed one allele to the offspring
(First Generation)= one G and one g!
Offspring 1- Heterozygous offspring (Gg)
 Mendel didn’t just stop after crossing the parent plants,
he had another question…
 Had the recessive alleles simply disappeared,
or were they still present in the new plants?
 P- Parent Generation
 F1- First (filial) Generation
 F2- Second (filial) Generation
• The F2 cross shows the recessive alleles
reappeared in the second generation. HOW?
• The Reappearance indicated that, at some point, the
allele for shortness had separated or segregated from
the allele for tallness. HOW?
 The alleles for tallness and shortness in the F1 plants
must have segregated from each other during the
formation of the SEX CELLS, or GAMETES.
P Generation= Tall plant (TT) X short plant (tt)
F1 Generation=All tall plants with genotype (Tt)
F1 (Tt) X F1 (Tt)
F2 Generation=
1 Tall plant (TT), 2 Tall plants with (Tt), 1 SHORT
plant with (tt)
During GAMETE
FORMATION, the alleles
for each gene segregate
from each other, so that
each gamete carries only
one allele for each gene.
 Mendel’s Principles of Heredity, observed through Patterns of Inheritance, form the
BASIS OF MODERN GENETICS.
1. The Inheritance of biological characteristics is determined by individual units called
GENES, which are passed from parent to offspring.
2. Where two or more form (alleles) of the gene for a single trait exist, some alleles may be
DOMINANT and others may be RECESSIVE.
3. In most sexually reproducing organisms, each adult has TWO COPIES of each gene—one
from each parent. These genes SEGREGATE from each other when gametes are formed.
4. Alleles for different genes usually SEGREGATE INDEPENDENTLY of each other.
 Many Organisms, including humans, reproduce sexually.
• They receive genes from both their parents
• Both parents contribute genes for the same traits.
The Genes may be the same, or they may code for different forms of
a trait.
Gene= height
Trait (allele)= short or tall
Alleles- are different forms of the gene for a specific
trait.
Genotype:
• Complete set of genes carried by
an organism
• Includes all the alleles that are
not expressed as well as those
that are.
• Ex: Rabbit Fur genotype can
include black and brown fur (Bb)
Phenotype:
• Set of traits that an
organism displays
• What you actually express
physically
• Ex: Rabbit Fur Phenotype
for (Bb) would be black fur.
Dominant Traits
 When an organism has two different alleles for a
trait, the dominant allele is expressed
(phenotype).
 Uppercase letters
 EX: Black Fur in rabbits BB (two dominant
Traitalleles)
Dominant Recessive
Freckles
Present
Absent
Hairline
Widow’s
peak
Straight
Earlobe
Free
Attached
Ability to taste PTC
Tasting
(phenylthiocarbamide)
Nontasting
Recessive Traits
 Are expressed only when no dominant alleles
are present.
 Lowercase letters
 EX: Brown fur in rabbits bb (two recessive
alleles)
 Organism that receives two identical
alleles for a characteristic shows
that characteristic (phenotype).
 Homozygous- two
dominant or two recessive
(BB or bb).
 Heterozygous- two
different alleles for a trait
(Bb).
Dominant And
Recessive
Traits
Phenylthiocarbamide has the
unusual property that it either
tastes very bitter or is virtually
tasteless, depending on the genetic
makeup of the taster.
Useful for finding the PROBABILITY of a simple genetic cross.
• Parent’s alleles are written across the top and side of the
square.
• Combining these alleles give the POSSIBLE genotypes of the
offspring
PRACTICE:
Create a Punnett Square in your notes:
1. Mom has Blue eyes (bb)
2. Dad has dark brown eyes (Bb)
 What is the possible genotypes and
phenotypes of their unborn baby?
MOM
DAD
Monohybrid Cross
Dihybrid Cross
(One-Factor Cross) (Two-Factor Cross)
Step 1: Write the Genotypes
for the parents
Ex: Pea Plant Height (Tt)
and/or Color (Gg)
Tt and Tt
TtGg and TtGg
Step 2: What alleles would be
found in all possible gametes
of the parents
Tt—T and t
Tt—T and t
TtGg—TG, Tg, tG, and tg
TtGg—TG, Tg, tG, and tg
Step 3: Draw a table with
enough squares for each pair
of gametes from each parent
Step 4: Fill in the table by
combining the gametes’
genotypes.
Step 5: Determine the
genotype and phenotype of
each offspring. Calculate the
percentage or ratio of each.
Incomplete Dominance- the result is a
BLEND of the two forms of the trait
Ex: Flower color
Codominance- Condition in which both
alleles are expressed in the same
organism.
Ex: Chicken Feathers & ABO Blood group
Multiple Alleles- although each organism has only two alleles for the
trait, more than two possible alleles may exist in the population.
Ex: Human Blood Types (A, B, AB, or O)
Polygenic Traits- are controlled by two or more genes
Ex: Height in humans
Blood Transfusions
Blood Type
of Donor
Blood Type of Recipient
A
B
AB
O
A
YES
X
YES
X
B
X
YES
YES
X
AB
X
X
YES
X
O
YES
YES
YES
YES
Sex-Linked Inheritance- Because X and Y
chromosome determines sex, the genes located on them
show a pattern of inheritance.
Sex-linked gene- gene located on a sex chromosome
• Genes on the Y chromosome are found ONLY in MALES
and are passed directly from father to son.
• Genes located on the X chromosome are found in both
sexes, but the fact that men have just one X chromosome
leads to some interesting consequences.
• Ex: three genes responsible for color vision, all
located on X chromosome
• Males- defective allele for any of these genes results
in colorblindness
• In order for a RECESSIVE allele to be EXPRESSED in
FEMALES, it must be present in TWO copies—one on
each of the X chromosomes.
 RECESSICE PHENOTYPES of a Sex-Linked genetic disorders
tends to be much more COMMON among MALES than
among females.
Shows the presence or absence of a trait according to
the relationships between parents, siblings, and
offspring.
• By analyzing a pedigree, we can often GUESS the genotypes of family members.
• Based on a pedigree, you can often determine if an allele for a trait is dominant or
recessive, autosomal or sex-linked.
“It runs in the family”
What does that really mean?
• Changes in a gene’s DNA sequence can change proteins by altering their
amino acid sequences, which may directly affect one’s phenotype.
 Disorders Caused by individual Genes:
 Sickle Cell Disease:
o Disorder caused by a defective allele for betaglobin, one of two polypeptides in hemoglobin.
o The defective polypeptide makes HEMOGLOBIN less
soluble, causing them to STICK TOGETHER.
o This causes the cell to get a SICKLE-SHAPE.
o Because of its shape it gets stuck in the capillaries
 Huntington’s Disease:
o Caused by a dominant allele for a protein found in
brain cells.
o The allele for this disease contains a long string of
bases in which the codon CAG (which codes for
Glutamine) repeats over and over again.
o Symptoms include mental deterioration and
uncontrollable movements
Central Dogma of Biology
 DNAmRNAprotein
 DNA TRANSCRIBES to mRNA
 Process is called transcription
 mRNA TRANSLATES to proteins
 Process is called translation
 mRNA actually makes amino acids, which come together
to make proteins
 RNA
 Single Strand
 Ribose sugar
 A=U
 G=C
 Uracil is the nitrogenous base
used instead of THYMINE
 DNA
 Double strand
 Deoxyribose sugar
 A=T
 G=C
DNA Replication Simplified
Unzip parent DNA
Add nucleotides to the 2 template strands of DNA
 DNA parent strand makes 2 daughter strands…one fast, smart daughter strand (leading) and one, slower, nofast daughter strand (lagging)
 Leading strand (runs 3’ to 5’)
 Lagging strand (runs 5’ to 3’)
Attach fragments on lagging strand
 Enzymes
 Make covalent bonds between nucleotides of the new strands
 Fast, accurate process
 Error only one in a billion nucleotides
 Brings over nucleotides to unzipped
DNA strand and drops them off
 DNA polymerase can only read a strand
that is running 3-prime to 5-prime…
 DNA polymerase works non-stop
adding nucleotides onto the strand
that runs in the 3’ to 5’ direction
 Therefore, Only one strand is made by
a smooth, and continuous process…
 The other strand is put together in bits
and pieces…
 Each little section of nucleotides is
called an “Okazaki Fragment”
 These are then “glued” together to
make one, continuous strand in the
end by another enzyme… DNA
Ligase
are short, newly synthesized DNA fragments that are
formed on the lagging template strand during DNA
replication.
DNA Helicase
unzips
DNA Polymerase
Adds nucleotides
DNA Ligase
Attaches/glues okazaki
 DNA codes for an RNA strand
 The every 3 bases on the RNA strand
code for a specific amino acid
 CODON: three sequential bases that
code for a specific a.a. (20 a.a. total)
 Amino acid are strung together to
make a protein (primary structure)
 Change DNA will change RNA which
will change amino acids, which change
protein
Ala: Alanine
Phe: Phenylalanine
Lys: Lysine
Pro: Proline
Thr: Threonine
Cys: Cysteine
Gly: Glycine
Leu: Leucine
Gln: Glutamine
Val: Valine
Asp: Aspartic acid
His: Histidine
Met: Methionine
Arg: Arginine
Trp: Tryptophane
Glu: Glutamic acid
Ile: Isoleucine
Asn: Asparagine
Ser: Serine
Tyr: Tyrosisne
 Transcription
 Different form of the same message
 DNA makes single stranded RNA (U replaces T)
 RNA leaves nucleus
 Translation
 Translate from nucleic acid language to amino acid language
 Uses codons, 3-base “word” that codes for specific a.a.
 “code” for an amino acid
 Several codons make a “sentence” that translates to a
polypeptide (protein)
Am. Biochemist Marshall
Nirenberg began to crack the
genetic code in the 1960s
 Built RNA model with Uracil, called poly U, conducted
experiments with it and figured out UUU coded for amino acid
phenylalanine
 Scientists used his procedures to figure out the other amino
acids represented by codons
Stop codons: UAA, UGA, UAG
 SIGNAL END OF GENETIC MESSAGE
Start codon: AUG
 SIGNAL TO START TRANSLATING an RNA transcript
Stop
Codons
 AUG
 UAA
 UGA
 UAG
mRNA
tRNA
rRNA
mRNA (messanger RNA)
 RNA transcribed from DNA template
 RNA polymerase (enzyme) links RNA nucleotides together
 Modified in nucleus before if exits
 RNA splicing: process in which Introns are removed and exons re joined together to make a
continuous coding mRNA molecule
 Introns
 Internal non-coding regions of DNA and mRNA
 Space fillers
 They are cut out of mRNA before it is allowed to leave the nucleus
 Process is called RNA splicing (processing)
 Exons
 Coding region of DNA and mRNA that will be translated (Expressed)
 VERY important part of mRNA…it is carrying the message from DNA (def can’t cut this out)
 tRNA (transfer RNA)
 The interpreter
 Translate 3-letter base codes into
amino acids
 Carries anti-codon on one end
(three letters opposite of what is
on mRNA)
 Carries amino acid on other end
 Anti-codon recognizes codon and
attaches
rRNA (ribosomal RNA)
 Found in ribosome
 Ribosome composed of 2 subunits:
 Small subunit for mRNA to attach
 Large Subunit for two tRNAs to
attach
 “P” site: holds the tRNA carrying
the growing polypeptide chain
 “A” site: holds the tRNA that is
carrying the next a.a. to be
added to the chain
 When stop codon (UAA, UAG, UGA) is reached, translation
ends and polypeptide is released from tRNA by hydrolysis